1. Field of the Invention
The present invention relates to an ultrasonic probe to be used for transmitting and receiving ultrasonic waves in an ultrasonic diagnostic apparatus. The present invention further relates to a backing material to be used for absorbing unwanted ultrasonic waves in the ultrasonic probe, and a method of manufacturing the backing material.
2. Description of a Related Art
An ultrasonic diagnostic apparatus transmits ultrasonic waves to an object to be inspected such as a human body or structure and receives ultrasonic echoes reflected from the object by using an ultrasonic probe, and displays images based on detection signals of ultrasonic waves. Thereby, examinations of organs and blood vessels within a body and nondestructive inspections within a structure are performed.
In the ultrasonic probe, as an ultrasonic transducer for transmitting and/or receiving ultrasonic waves, a vibrator (piezoelectric vibrator) having electrodes formed on both sides of a piezoelectric material such as piezoelectric ceramics represented by PZT (Pb(lead) zirconate titanate), a polymeric piezoelectric material represented by PVDF (polyvinylidene difluoride) is generally used.
When a voltage is applied to the electrodes of the vibrator, the piezoelectric material expands and contracts due to the piezoelectric effect and generates elastic waves. Further, plural vibrators are one-dimensionally or two-dimensionally arranged and driven by plural drive signals with predetermined delays, and thereby, an ultrasonic beam can be formed in a desired direction. On the other hand, the vibrators expand and contract when receiving propagating ultrasonic waves and generate electric signals. The electric signals are used as detection signals of the ultrasonic waves.
Generally, the vibrators are formed on a backing material and an acoustic matching layer and, according to need, an acoustic lens are further formed on the vibrators. The backing material has a function of absorbing unwanted ultrasonic waves to be emitted from the backsides of the vibrators. If the backing material has poor acoustic characteristics, it cannot sufficiently absorb unwanted ultrasonic waves but may cause degradation of image quality of ultrasonic waves.
Further, when plural vibrators are one-dimensionally or two-dimensionally arranged in an arrayed structure, the wiring becomes complicated. In this regard, it is conceivable that the structure of the ultrasonic probe is compactly configured by forming a signal wiring structure on the side or within the backing material. In this case, if the backing material has conductivity, the wiring may be shorted or noise may be mixed. Therefore, the backing material requires an insulation property.
In order to suppress the reflection of ultrasonic waves at an interface between the vibrators and the backing material and attenuate the transmitted ultrasonic waves, it is desirable that the value of the acoustic impedance of the backing material is closer to the value of the acoustic impedance of the vibrators. Further, the acoustic matching layer matches the acoustic impedance of the vibrators with the acoustic impedance of a human body or the like and reduces the reflection of ultrasonic waves.
The acoustic impedance is a constant intrinsic to a material as expressed by equations (1) or (2), and the unit of MRayl (mega Rayl) is generally used therefor, and 1MRayl =1×106kg·m−2·s−1.Z=ρ·v   (1)Z=(ρ·K)1/2   (2)where “ρ” represents density of an acoustic medium, “v” represents acoustic velocity within the acoustic medium, and “K” represents a bulk modulus of the acoustic medium. The acoustic impedance of typical piezoelectric ceramics is about 25MRayl to 35MRayl, while the acoustic impedance of a human body is about 1.5MRayl.
Given that the acoustic impedance of the vibrator is Z1 and the acoustic impedance of the backing material is Z2, the vertical reflectance of ultrasonic waves at the interface between the vibrator and the backing material is given by the following equation (3).IR/I0=|Z2−Z1|/(Z2+Z1)  (3)where I0 represents acoustic pressure of ultrasonic waves entering the interface and IR represents acoustic pressure of ultrasonic waves reflected at the interface.
Further, the vertical transmittance of ultrasonic waves at the interface between the vibrator and the backing material is given by the following equation (4).IT/I0=2·Z2/(Z2+Z1)  (4)where IT represents acoustic pressure of ultrasonic waves transmitted through the interface.
A material having a large attenuation rate like rubber as a kind of elastomer (elastic polymer compound) is desirable for the backing material. However, if only rubber is used, the acoustic impedance of the backing material becomes as small as about 1MRayl because the rubber has low density and low balk modulus. For this reason, the acoustic impedance is made higher by dispersing powder of an inorganic material having high specific gravity in an elastomer or resin to make composition.
As an inorganic material having high specific gravity, tungsten (W; density 19,200 kg/m3), tantalum (Ta; density 16,700 kg/m3), gold (Au; density 19,300 kg/m3), platinum (Pt; density 21,100 kg/m3), iridium (Ir; density 22,700 kg/m3), tungsten carbide (WC; density 15,600 kg/m3), tantalum carbide (TaC; density 14,500 kg/m3), tungsten silicide (WSi2), or the like is used.
However, all of these materials are conductive and, if the filling rate of the powder is made higher, the backing material itself becomes conductive. That is, percolation conduction occurs, in which many powder particles filled therein contact one another and form conduction paths. Further, since the materials are transition metals or partially contain transition metals, the cross-linking and curing reaction of the elastomer or resin becomes unstable due to the catalyst effect of the transition metals, and therefore, it is difficult to obtain a backing material having homogeneous acoustic characteristics.
As a related technology, Japanese Patent Application Publication JP-P2003-190162A discloses a method of manufacturing a backing for an ultrasonic probe in order to improve acoustic property of the backing. The method of manufacturing a backing includes the first step of manufacturing plural compounds and the second step of manufacturing a backing by mixing the plural compounds in a parent material, and the first step includes an adding step of adding a second material and a third material to a first material having fluidity, a curing step of curing the first material to which the second material and the third material have been added to produce an original compound material, and a step of crushing the original compound material to produce plural compounds.
However, as shown in FIG. 1 of JP-P2003-190162A, particles 28 of the second material having conductivity such as tungsten are exposed on the surface of compounds 24 manufactured in the first step, and, if the filling rate of the compounds 24 is made higher, percolation conduction occurs. On the other hand, if the filling rate of the compounds 24 is lower, it is impossible to make the acoustic impedance of the backing higher. Further, since the particles 28 containing a transition metal such as tungsten are exposed, the curing reaction of a parent material 22 or a base material 26 such as silicone rubber or urethane resin becomes inhomogeneous due to their catalyst effect. That is, the curing reaction quickly occurs at the part in contact with the particles 28, and inhomogeneous filling rate and air bubble mixture are caused. Furthermore, since the composite material is produced and crushed to manufacture the plural compounds, and then, those compounds are mixed in the parent material, the manufacturing process of the backing becomes complicated.